The present invention relates to an inspection system for inspecting an object, e.g. a wafer, by using an electron beam to detect a defect or the like in a pattern formed on the surface of the sample under inspection. More particularly, the present invention relates to an inspection system and inspection method wherein an electron beam is irradiated to the surface of an object under inspection, and image data is obtained from the number of secondary electrons emitted from the sample surface, which varies according to the properties of the sample surface, and a pattern or the like formed on the sample surface is inspected with high throughput on the basis of the image data, as in the case of detecting defects on a wafer in the semiconductor fabrication process. The present invention also relates to a method of fabricating devices at high yield by using the inspection system.
Semiconductor fabrication processes are going to enter a new era where design rules are 100 nm. The form of production is shifting from limited large-lot production, represented by the fabrication of DRAMs, to diverse small-lot production as is the case with SOC (Silicon On Chip). As a result, the number of production steps has increased, and it has become essential to improve the yield for each production step. Consequently, the inspection for process-induced defects has become important. Conventionally, a wafer defect inspection is performed after each step of the semiconductor manufacturing process. With the progress of the technology to fabricate high-integration semiconductor devices and to form small and fine patterns, a high-resolution and high-throughput defect inspection system has been demanded. The reason for this is that a resolution of 100 nm or below is required to detect the defects on a wafer substrate fabricated with 100 nm design rules. In addition, as the degree of integration of semiconductor devices increases, the number of production steps increases, resulting in an increase in the number of inspections to be performed. For this reason, high throughput is demanded. Further, as the number of layers constituting semiconductor devices increases, it is required that the defect inspection system should have the function of detecting contact failures (electrical defects) of vias for connection between wiring patterns on different layers.
As this type of defect inspection system, an optical defect inspection system has heretofore been used. However, the optical defect inspection system is limited in its resolution capability. That is, the resolution is limited to ½ of the wavelength of light used. In a practical example using visible light, the resolution is of the order of 0.2 μm. Thus, the optical defect inspection system cannot meet the resolution requirements. Further, the optical defect inspection system cannot perform inspection for electrical conduction failures (opens, shorts, etc.), that is, contact failures occurring in semiconductor devices.
Under these circumstances, a defect inspection system using an electron beam has recently been developed for use in place of the optical defect inspection system.
In such an electron beam type defect inspection system, a scanning electron microscope system (SEM system) has generally been put to practical use. The inspection system exhibits a relatively high resolution, i.e. 0.1 μm, and is capable of inspection for electrical defects (disconnection and conduction failures of wiring patterns, conduction failures of vias, etc.). In the defect inspection system making use of SEM, however, the amount of beam current and the response speed of the detector are limited. Therefore, a great deal of time is required to perform defect inspection. For example, it takes 8 hours to inspect one wafer (20-cm wafer). Thus, the inspection time is extremely long. Accordingly, the throughput (the number of wafers inspected per unit time) is unfavorably low in comparison to other process systems such as optical defect inspection systems. In addition, the electron beam type defect inspection system is very costly. Accordingly, it is difficult to use it after each step of the semiconductor fabrication process. In the present state of the art, the electron beam type defect inspection system is used after an important process step, e.g. after etching, film deposition (including copper plating), or CMP (Chemical/Mechanical Polishing) planarization treatment.
The defect inspection system using the scanning electron microscope system (SEM system) will be described below in more detail. In the defect inspection system, an electron beam is focussed (the focussed beam diameter corresponds to the resolution) and a sample, e.g. a wafer is linearly irradiated so as to be scanned with the focussed electron beam. Meanwhile, a stage having the wafer placed thereon is moved in a direction perpendicular to the electron beam scanning direction, whereby an observation region on the wafer is irradiated planarly with the electron beam. The scanning width of the electron beam is generally several 100 μm. By the irradiation with the focussed electron beam (referred to as “primary electron beam”), secondary electrons are emitted from the wafer. The secondary electrons are detected with a detector (a scintillator+a photomultiplier) or a semiconductor type detector (a PIN diode type detector), for example. The coordinates of the position of irradiation with the electron beam and the number of secondary electrons (signal intensity) are combined to produce an image. Image data thus obtained is stored in a storage unit. Alternatively, the image can be output onto a CRT (Cathode-Ray Tube). The foregoing is the principle of the SEM (Scanning Electron Microscope). From the image obtained by this method, possible defects on the in-process semiconductor (Si, in general) wafer are detected. The inspection speed (corresponding to the throughput) is determined by the amount of the primary electron beam (electric current value), the beam diameter, and the response speed of the detector. The present maximum values of these factors are as follows. The beam diameter is 0.1 μm (this may be regarded as equal to resolution). The electric current value is 100 nA. The response speed of the detector is 100 MHz. In this case, it takes about 8 hours to inspect one wafer having a diameter of 20 cm, as has been stated above. Thus, the scanning electron beam type defect inspection system suffers from a serious problem that the inspection speed is extremely lower ( 1/20 or less) than those of other process systems such as optical defect inspection systems.
The present invention was made in view of the above-described problems. Accordingly, an object of the present invention is to improve the inspection speed for detecting defects on a sample, e.g. a wafer.